Login to Dbconsole, Authentication failed!null Returned

Today I faced this error whenever I stop the listener service of my oracle (Version 10.2.0.1) and I attempt to connect to oracle Enterprise manager it returned with the error.

Login to Dbconsole, Authentication failed!null Returned

But I could not reproduce this error. Later on, whenever I tried to reproduce the error , every time I stop listener and try to connect to database using Enterprise Manager it returned with the error,

Error
Io exception: The Network Adapter could not establish the connection

Solution of these Two Problem:
----------------------------------
Your listener service is down but your enterprise manager is running. So start the listener.

From my UNIX machine,

SQL>!lsnrctl start
SQL>alter system register;

And I log on through Enterprise Manager and it is running fine.

See Also http://arjudba.blogspot.com/2008/08/authentication-failednull.html